Random Nerd Tutorials

ESP32 Web Server using SPIFFS (SPI Flash File System)


In this tutorial we’ll show you how to build a web server that serves HTML and CSS files stored on the ESP32 filesystem. Instead of having to write the HTML and CSS text into the Arduino sketch, we’ll create separated HTML and CSS files. For demonstration purposes, the web server we’ll build controls an ESP32 output, but it can be easily adapted for other purposes like displaying sensor readings.

ESP32 Filesystem Uploader Plugin

To follow this tutorial you should have the ESP32 Filesystem Uploader plugin installed in your Arduino IDE. If you haven’t, follow the next tutorial to install it first:

Note: make sure you have the latest Arduino IDE installed, and you have the ESP32 add-on for the Arduino IDE. If you don’t, follow one of the following tutorials to install it:

Project Overview

Before going straight to the project, it’s important to outline what our web server will do, so that it is easier to understand.

  • The web server you’ll build controls an LED connected to the ESP32 GPIO 2. This is the ESP32 on-board LED. You can control any other GPIO;
  • The web server page shows two buttons: ON and OFF – to turn GPIO 2 on and off;
  • The web server page also shows the current GPIO state.

The following figure shows a simplified diagram to demonstrate how everything works.

  • The ESP32 runs a web server code based on the ESPAsyncWebServer library;
  • The HTML and CSS files are stored on the ESP32 SPIFFS (Serial Peripheral Interface Flash File System);
  • When you make a request on a specific URL using your browser, the ESP32 responds with the requested files;
  • When you click the ON button, you are redirected to the root URL followed by /on and the LED is turned on;
  • When you click the OFF button, you are redirected to the root URL followed by /off and the LED is turned off;
  • On the web page, there is a placeholder for the GPIO state. The placeholder for the GPIO state is written directly in the HTML file between % signs, for example %STATE%.

Installing Libraries

In most of our projects we’ve created the HTML and CSS files for the web server as a String directly on the Arduino sketch. With SPIFFS, you can write the HTML and CSS in separated files and save them on the ESP32 filesystem.

One of the easiest ways to build a web server using files from the filesystem is by using the ESPAsyncWebServer library. The ESPAsyncWebServer library is well documented on its GitHub page. For more information about that library, check the following link:

Installing the ESPAsyncWebServer library

Follow the next steps to install the ESPAsyncWebServer library:

  1. Click here to download the ESPAsyncWebServer library. You should have a .zip folder in your Downloads folder
  2. Unzip the .zip folder and you should get ESPAsyncWebServer-master folder
  3. Rename your folder from ESPAsyncWebServer-master to ESPAsyncWebServer
  4. Move the ESPAsyncWebServer folder to your Arduino IDE installation libraries folder

Installing the Async TCP Library for ESP32


The ESPAsyncWebServer library requires the AsyncTCP library to work. Follow the next steps to install that library:

  1. Click here to download the AsyncTCP library. You should have a .zip folder in your Downloads folder
  2. Unzip the .zip folder and you should get AsyncTCP-master folder
  3. Rename your folder from AsyncTCP-master to AsyncTCP
  4. Move the AsyncTCPfolder to your Arduino IDE installation libraries folder
  5. Finally, re-open your Arduino IDE

Organizing your Files

To build the web server you need three different files. The Arduino sketch, the HTML file and the CSS file. The HTML and CSS files should be saved inside a folder called data inside the Arduino sketch folder, as shown below:

Creating the HTML File

The HTML for this project is very simple. We just need to create a heading for the web page, a paragraph to display the GPIO state and two buttons.

Create an index.html file with the following content or download all the project files here:

Because we’re using CSS and HTML in different files, we need to reference the CSS file on the HTML text. The following line should be added between the <head> </head> tags:

<link rel="stylesheet" type="text/css" href="style.css">

The <link> tag tells the HTML file that you’re using an external style sheet to format how the page looks. The rel attribute specifies the nature of the external file, in this case that it is a stylesheet—the CSS file—that will be used to alter the appearance of the page.

The type attribute is set to “text/css” to indicate that you’re using a CSS file for the styles. The href attribute indicates the file location; since both the CSS and HTML files will be in the same folder, you just need to reference the filename: style.css.

In the following line, we write the first heading of our web page. In this case we have “ESP32 Web Server”. You can change the heading to any text you want:

<h1>ESP32 Web Server</h1>

Then, we add a paragraph with the text “GPIO state: ” followed by the GPIO state. Because the GPIO state changes accordingly to the state of the GPIO, we can add a placeholder that will then be replaced for whatever value we set on the Arduino sketch.

To add placeholder we use % signs. To create a placeholder for the state, we can use %STATE%, for example.

<p>GPIO state: <strong>%STATE%</strong></p>

Attributing a value to the STATE placeholder is done in the Arduino sketch.

Then, we create an ON and an OFF buttons. When you click the on button, we redirect the web page to to root followed by /on url. When you click the off button you are redirected to the /off url.

<p><a href="/on"><button class="button">ON</button></a></p>
<p><a href="/off"><button class="button button2">OFF</button></a></p>

Creating the CSS file

Create the style.css file with the following content or download all the project files here:

This is just a basic CSS file to set the font size, style and color of the buttons and align the page. We won’t explain how CSS works. A good place to learn about CSS is the W3Schools website.

Arduino Sketch

Copy the following code to the Arduino IDE or download all the project files here. Then, you need to type your network credentials (SSID and password) to make it work.

How the Code Works

First, include the necessary libraries:

#include "WiFi.h"
#include "ESPAsyncWebServer.h"
#include "SPIFFS.h"

You need to type your network credentials in the following variables:

const char* ssid = "REPLACE_WITH_YOUR_SSID";
const char* password = "REPLACE_WITH_YOUR_PASSWORD";

Next, create a variable that refers to GPIO 2 called ledPin, and a String variable to hold the led state: ledState.

const int ledPin = 2;
String ledState;

Create an AsynWebServer object called server that is listening on port 80.

AsyncWebServer server(80);


The processor() function is what will attribute a value to the placeholder we’ve created on the HTML file. It accepts as argument the placeholder and should return a String that will replace the placeholder. The processor() function should have the following structure:

String processor(const String& var){
  if(var == "STATE"){
      ledState = "ON";
      ledState = "OFF";
    return ledState;
  return String();

This function first checks if the placeholder is the STATE we’ve created on the HTML file.

if(var == "STATE"){

If it is, then, accordingly to the LED state, we set the ledState variable to either ON or OFF.

  ledState = "ON";
  ledState = "OFF";

Finally, we return the ledState variable. This replaces the placeholder with the ledState string value.

return ledState;


In the setup(), start by initializing the Serial Monitor and setting the GPIO as an output.

pinMode(ledPin, OUTPUT);

Initialize SPIFFS:

  Serial.println("An Error has occurred while mounting SPIFFS");

Wi-Fi connection

Connect to Wi-Fi and print the ESP32 IP address:

WiFi.begin(ssid, password);
while (WiFi.status() != WL_CONNECTED) {
  Serial.println("Connecting to WiFi..");

Async Web Server

The ESPAsyncWebServer library allows us to configure the routes where the server will be listening for incoming HTTP requests and execute functions when a request is received on that route. For that, use the on() method on the server object as follows:

server.on("/", HTTP_GET, [](AsyncWebServerRequest *request){
  request->send(SPIFFS, "/index.html", String(), false, processor);

When the server receives a request on the root “/” URL, it will send the index.html file to the client. The last argument of the send() function is the processor, so that we are able to replace the placeholder for the value we want – in this case the ledState.

Because we’ve referenced the CSS file on the HTML file, the client will make a request for the CSS file. When that happens, the CSS file is sent to the client:

server.on("/style.css", HTTP_GET, [](AsyncWebServerRequest *request){
  request->send(SPIFFS, "/style.css","text/css");

Finally, you need to define what happens on the “/on” and “/off” routes. When a request is made on those routes, the LED is either turned on or off, and the ESP32 serves the HTML file.

server.on("/on", HTTP_GET, [](AsyncWebServerRequest *request){
  digitalWrite(ledPin, HIGH);
  request->send(SPIFFS, "/index.html", String(),false, processor);
server.on("/off", HTTP_GET, [](AsyncWebServerRequest *request){
  digitalWrite(ledPin, LOW);
  request->send(SPIFFS, "/index.html", String(),false, processor);

In the end, we use the begin() method on the server object, so that the server starts listening for incoming clients.


Because this is an asynchronous web server, you can define all the requests in the setup(). Then, you can add other code to the loop() while the server is listening for incoming clients.

Uploading Code and Files

Save the code as Async_ESP32_Web_Server or download all the project files here. Go to Sketch > Show Sketch Folder, and create a folder called data. Inside that folder you should save the HTML and CSS files.

Then, upload the code to your ESP32 board. Make sure you have the right board and COM port selected. Also, make sure you’ve added your networks credentials to the code.

After uploading the code, you need to upload the files. Go to Tools > ESP32 Data Sketch Upload and wait for the files to be uploaded.

When everything is successfully uploaded, open the Serial Monitor at a baud rate of 115200. Press the ESP32 “ENABLE” button, and it should print the ESP32 IP address.


Open your browser and type the ESP32 IP address. Press the ON and OFF buttons to control the ESP32 on-board LED. Also, check that the GPIO state is being updated correctly.

Wrapping Up

Using SPI Flash File System (SPIFFS) is specially useful to store HTML and CSS files to serve to a client – instead of having to write all the code inside the Arduino sketch.

The ESPAsyncWebServer library allows you to build a web server by running a specific function in response to a specific request. You can also add placeholders to the HTML file that can be replaced with variables – like sensor readings, or GPIO states, for example.

If you liked this project, you may also like:

This is an excerpt from our course: Learn ESP32 with Arduino IDE. If you like ESP32 and you want to learn more, we recommend enrolling in Learn ESP32 with Arduino IDE course.

Learn ESP32 with Arduino IDE

This is our complete guide to program the ESP32 with Arduino IDE, including projects, tips, and tricks! The registrations are open, so SIGN UP NOW »

Recommended Resources

Home Automation using ESP8266 »
Build IoT projects and home automation gadgets with the ESP8266 Wi-Fi module.

Build a Home Automation System »
Learn how to build a automation system using open-source hardware and software from scratch.

Arduino Step-by-Step Projects »
Build 25 cool Arduino projects with our course even with no prior experience!

Leave a Comment:

Add Your Reply

Enroll in our Electronics and Programming Courses:

ESP32, ESP8266, Arduino and much more.